1. Field of the Invention
The present invention relates to an angular encoder including a flexible measurement tape having a graduation and secured to a sector of a convex or concave, as the case may be, circumferential surface of a support, and a scanning head for scanning the graduation of the measurement tape.
2. Description of the Prior Art
In angular encoders of the above-described type, the flexible measurement tape is glued to the circumferential surface of the support. At that, the tape is fixedly held at one end and is wrapped around an appropriate region of the convex or concave support surface. The friction between the support surface and the measurement tape leads to different stress ratios along the tape. The stress at the first, fixedly secured end is minimal, as a result of a stationary friction, and the stress increases toward the other end of the measurement tape. The deviation of the incremental graduation changes along the measurement tape in accordance with the changes of the stress ratios.
A measurement tape with the unfavorable stress ratios is disclosed, e.g., in German Publication DE 37 15 908 A1. In the encoder disclosed in this publication, a flexible measurement tape has its both ends secured on convexly curved support with magnetic holders or with suction cups. The present invention is directed to an improvement of the angular encoder disclosed in DE 37 15 908 A1.
Alfons Ernst in his book "Digitale Langen-und Winkel Messtechnik" (Digital Linear and Angular Measurement Technology), Technical Library, Volume 34, 1989, p.p. 60-61, describes an angular encoder in which a steel tape with an incremental graduation is tensioned around a circumference of a rotary table. The tape ends are welded to clamping cleats which are formed so precisely that upon mounting the tape on the rotary table, they need only be screwed together. The tensioning cleats insure a precise abutment of the tape ends with each other. However, this securing of a measurement tape is possible only about a circumference of 360.degree..
Several publications, e.g., German Patent No, 2,712,421 and European Publications EP 0 105 119 B1 and EP 0 169 189 B1, disclose a linear encoder in which a flexible measurement tape is fixed to a support at one of its ends, with the other end being tensioned with a tensioning device. In these encoders, the tensioning device includes a spring and a straining screw for adjusting the tape length.
German Utility model DE 79 07 900 discloses a linear encoder including a linear elongate scale at each of the opposite ends of which a correction device is provided. The correction device can be formed as a stretching or tensioning device or as a submerging device. The measurement tape in this encoder is supported on a flat surface of a hollow body.
In linear encoders, no wrapping-around of a tape takes place, which influence the stress ratios dependent on a wrap-around angle.
Accordingly, an object of the present invention is to provide an angular encoder with a flexible measurement tape supported on a convexly or concavely curved support and which is easily mountable thereon.
Another object of the present invention is to provide an angular encoder with a flexible measurement tape in which the friction ratios and, thus, the stress and tension ratios of a measurement tape are significantly improved in comparison with known solutions.